With line-commutated converters, as are described for example in AT 404 414 B written by the applicant, faults in the voltage or current waveform, especially overvoltages and/or commutation errors, lead to a destruction of the expensive thyristors or switches of such a bridge.
In more precise terms the basic problem which arises with said line-commutated converters is that of shoot-through which occurs in feedback operation. In such cases an overcurrent arises on power outage and sufficient dc voltage, e.g. a corresponding armature voltage of the motor, which further increases and is also no longer able to be quenched by the converter itself. The inevitable result is then a triggering of fuses which are generally provided and are intended to protect the thyristors. The consequence is a longer downtime of the converter and of the motor fed by it since an appropriate period of time is required to replace the fuses which are usually embodied as fast semiconductor fuses. Efforts were thus made to create various, sometimes complex devices, to limit and to interrupt the overcurrent or to either prevent a shoot-through or to end it in a controlled manner, such as in the above-mentioned patent of the applicant for example.
Fast direct current switches are known for example which lie in the direct current path. If their secure function is to be guaranteed, additional highly-controllable chokes are necessary to restrict the rise in the current and the costs are accordingly considerable, quite apart from the problematic dimensioning and regular maintenance required for secure operation.
Other methods for direct thyristor quenching with quenching capacitors are also known, for example the quenching of only one of the two halves of the converter bridge with a capacitor, whereby the motor current still fully loads the valves of the second half of the bridge until the motor current is completely reduced to zero and thereby protection is not possible in all cases. A similar method is the quenching of both bridge halves with two capacitors and with a voltage limitation in parallel to this in each case, which however allows overvoltages at the motor. In addition the stated methods do not protect against an overvoltage in the event of a power outage if there is a transformer between network and converter bridge.